1. Field of the Invention
This invention relates to a process for effectively preparing silicon nitride powder having a high alpha-type content.
2. Prior Art
In the prior art, silicon nitride powder is generally prepared by the direct nitriding reaction of metallic silicon powder in a non-oxidizing atmosphere containing nitrogen gas or ammonia gas at a temperature of 1,000 to 1,500.degree. C. This process, however, has the problem that once nitriding starts, a great amount of reaction heat is generated by the following reaction scheme: EQU 3Si+2N.sub.2 =Si.sub.3 N.sub.4 EQU .increment.H.sub.R =-176 kcal/mol
to rise the temperature of the reaction system so that the amount of beta-type silicon nitride powder which is stable at higher temperature is increased. As a consequence, it was difficult to prepare high alpha-type silicon nitride powder having an alpha content of higher than 90%.
Then in order to produce silicon nitride powder at a high alpha conversion rate, several approaches were taken including (i) addition of a catalyst as disclosed in Japanese Patent Application Kokai (JP-A) 15499/1979, (ii) slowing down the heating rate as disclosed in JP-A 24300/1979, and (iii) use of starting silicon powder with a finer particle size as disclosed in JP-A 41048/1983. Undesirably, the catalyst added in method (i) would be left in the nitride product as an impurity. Method (ii) suffered from low productivity. Method (iii) required a substantial cost in comminuting the starting metallic silicon powder.